Conventionally, polyolefin-based resins have been extensively used in a variety of applications in the form of various molded articles, for example, sheets, films, injection-molded articles, blow-molded articles, foam-molded articles, vacuum-molded articles and rotational-molded articles which are formed into desired shapes or desired properties by various molding methods, because the polyolefin-based resins are chemically stable, and excellent in weather resistance, chemical resistance and mechanical strength.
Also, with a recent increasing tendency toward environmental protection, there is such a steady demand that general plastics are required to show a less burden to environments. Among them, polyolefin-based resins have been noticed as resins most suitable for reducing the burden to environments because of their excellent recyclability, good moldability and formation of less harmful components upon burning. Therefore, it is expected that the polyolefin-based resins are more frequently used in future, and are required to show highly improved properties.
However, even if one kind of polyolefin-based material is used solely, it is not possible to achieve the highly improved properties or various excellent properties as required. Therefore, to meet the above requirements, it has been conventionally attempted to combine a plurality of polyolefin-based materials to provide a suitable composite material.
However, in general, since different kinds of polyolefin-based resins are immiscible with each other, the morphology of the obtained composite material is regulated only to a limited extent, so that such a composite material fails to exhibit excellent combined properties of the respective polyolefin-based resins as expected.
On the other hand, Japanese Patent Application Laid-open No. Hei 2-281012 discloses a method of polymerizing a vinyl-based monomer or a vinylidene-based monomer using a radical polymerization initiator in the presence of a copolymer of propylene and dialkenyl benzene. Further, in Examples of the above Japanese Patent Application Laid-open No. Hei 2-281012, it is described that the thus obtained polymer is used as a miscibilizer for polypropylene and polystyrene.
In the method described in the above Japanese Patent Application Laid-open No. Hei 2-281012, a radical polymerizable monomer is polymerized using a radical polymerization initiator to produce graft chains having an atactic structure. Thus, the above method fails to produce polymers having an isotactic or syndiotactic structure capable of showing a more excellent heat resistance. Further, in the above Japanese Patent Application Laid-open No. Hei 2-281012, although it is merely described that the miscibilizer is applied to a composite material composed of isotactic polypropylene and isotactic polystyrene, there is no concrete description concerning application of the miscibilizer to other composite materials.
In addition, Japanese Patent Application Laid-open No. Hei 3-28209 discloses a graft copolymer produced by graft-polymerizing propylene to an ethylene-α-olefin-diene copolymer and a resin composition containing the graft copolymer, and further describes effects obtained by adding the graft copolymer to polypropylene and EPR. However, as shown in Examples of Japanese Patent Application Laid-open No. Hei 3-28209, since the graft copolymer contains gel components insoluble in a solvent, the obtained composite material is improved in properties only to a limited extent, and tends to show insufficient surface properties and poor appearance. Further, since the graft polymerization is usually conducted using a titanium trichloride type catalyst, the copolymerizability between diene residues and propylene is extremely low, thereby failing to achieve a high grafting rate. Therefore, since a large amount of graft sites, i.e., carbon-to-carbon unsaturated bonds remain unreacted, the obtained graft copolymer itself is inevitably deteriorated in heat stability, so that physical properties, heat stability, etc., of the composite material produced from such a graft copolymer are controlled only to a limited extent.
Also, the use of the above titanium trichloride type catalyst leads to not only production of polypropylene having a broad molecular weight distribution, but also production of a large amount of components soluble in ether or the like (low-stereoregular, low-molecular weight components) as well as by-products that do not contribute to improvement in miscibility, thereby causing such a problem that the obtained composite material tends to be deteriorated in physical properties.